The present invention relates generally to alignment targets and more specifically to an alignment target having enhanced contrast.
Alignment targets generally used in integrated circuit fabrication generally include a series of indicia on the surface or subsurface of a substrate which are used to align masking patterns for photolithographic machinery. The alignment target is used for not only locating a particular position on the wafer, but is also used to align the wafer in two axes. The degree of accuracy of the alignment is a function of the resolution of the optical system. Similarly, lack of contrast between the alignment indicia or target does effect the accuracy and resolution.
One solution to the resolution is not to use a direct optical reading, but to use diffraction patterns as illustrated in U.S. Pat. Nos. 4,200,395 and 4,211,489.
Recognition of the problem of low target to background contrast is recognized by U.S. Pat. Nos. 4,103,998 and 4,390,279. The first patent solves this problem by using coherent light. In the second patent, the solution is to use a unique optical system to receive specific reflected light from the target.
Thus, there exists a need for a direct optical reading system of improved contrast.
It is an object of the present invention to provide an enhanced contrast alignment target for use in integrated circuits.
Another object of the present invention is to provide an alignment target of improved contrast for use in a digital optical system.
These and other objects of the invention are attained by providing an optical target which includes a highly reflective target region surrounded by a substantial contrast region having light scattering characteristics. The contrast region generally includes a plurality of hills and valleys having parallel axis with a mimimum of flat reflective area. Preferably, the parallel axis of the peaks and valleys are oblique to the scanning axis of the automatic optical system. Any flat area of the contrast region is substantially small along the scanning axis compared to the width of the target region along the scanning axis.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.